Her mother, Kim Young-ah is a piano teacher and taught Seohyun when she was 5. In 2009, Girls' Generation appeared on an episode of Kim Jungeun's Chocolate and as a special guest, Seohyun's mom joined the idols on stage.[2]

As an elementary school student, Seohyun was discovered by a talent staff while riding in a subway train, who encouraged her to audition for SM Entertainment. In 2003, Seohyun auditioned and joined the company under the SM Casting System. She is known for singing children's songs for her audition unlike other applicants who usually sing pop and dance songs.[3] She attended Seoul Middle School and Daeyoung High School, then transferred and graduated from Jeonju Arts High School on February 9, 2010.[4] On August 21, 2014, she graduated from Dongguk University,[5][6] receiving a degree in acting and an Achievement Award at the University's graduation ceremony.[7][8] Her Girls' Generation's bandmate Yoona also attended the same university.

Seohyun named United Nations Secretary-General Ban Ki-moon as her life mentor and role model, stating that his book, at one point in time, had helped her endure through the hard times.[9][10]

Seohyun started her training under SM Entertainment after joining the agency. In 2007, she was chosen to be a member of the girl group Girls' Generation. The 9-member girl group debuted on August 5, 2007.

Taeyeon, Tiffany and Seohyun formed a subgroup called TTS in 2012, releasing their first mini-album titled Twinkle during the same year. The subgroup has a different musical direction and style concept from the 9-member Girls' Generation,[11] placing their focus on each member's vocal ability while the latter group emphasizes the songs and uniform group performances.[12] In 2014, the group released their 2nd mini album, Holler.[13]

Over the life of her career, Seohyun has released several songs for various television dramas and movies. A couple of them are her solos, while others are collaborations with fellow Girls' Generation members and other artists.

Her solos include "It's Okay Even If It Hurts" for the 2010 MBC's drama Kim Soo Ro,[14] and "I'll Wait for You" for the 2012 SBS's Fashion King.[15]

Seohyun is the fifth member of Girls' Generation to be credited as a songwriter after Yuri, Sooyoung, Tiffany and Jessica.[26] In Girls' Generation's fourth Korean studio album, I Got a Boy, she co-wrote two songs "Baby Maybe" with fellow group members Yuri & Sooyoung and "XYZ" with Yuri.

Subsequently, in TTS's 2nd mini album Holler, Seohyun wrote the lyrics for the song "Only U". She expressed "The lyrics are about the sincerity of love, about a true love. That you fall in love many times, but there's always going to be that special someone you won't forget. And that I want to find that special someone, out of all those people you have to meet through, and keep them in my life forever."[27]

Seohyun and her bandmate Taeyeon were cast as voice actors for the Korean-dubbed versions of the computer-animated films Despicable Me and Despicable Me 2. Tae-yeon and Seohyun dubbed the voices for the characters Margo and her younger sister, Edith, respectively.[28][29] In 2013, Seohyun made her television acting debut in the SBS's drama Passionate Love. She played a supporting character, Han Yoo Rim, who is the first love of the male lead, portrayed by actor Sung Hoon.[30] She and her bandmate Sunny also served as the background narrators for the MBC variety show Real Men.[31]

Apart from her appearances with other Girls' Generation members on various shows, she partnered with singer and actor Jung Yong-hwa, who is the main singer of the idol band CN Blue, as a virtually married couple for MBC's TV show We Got Married. The two became known as the YongSeo Couple. The couple first met on 11 February 2010, and filmed their final episode on White Day, 14 March 2011, and left the show on April 2, 2011, when their final episode aired.

The TaeTiSeo, featuring Seohyun and fellow Girls' Generation members Taeyeon and Tiffany, premiered on August 26 and consisted of 8 episodes. The show took on a more close-up and personal side of the three singers, revealing how they live their normal everyday lives and what they do in their free time.[40] When speaking of their own subgroup, TTS, Seohyun stated "We are Girls' Generation, but I think that a musical color unique to only TaeTiSeo (TTS) is forming."[41][42]

Seohyun appeared as a replacement for Eugene in the Korean group S.E.S. who could not perform due to her pregnancy in episodes 409 through 411 of the variety show Infinity Challenge from 20 December until 3 January 2015 in their Saturday's: I am a Singer special.[43][44]

In 2010, Seohyun was appointed by the Seoul Metropolitan Office as the South Korea's Goodwill Ambassador for Student Fitness.[45] In 2011, she, who has previously worked with the UNICEF's “Help Children in Africa” project, was then inaugurated as a UNICEF envoy.[46] Subsequently, Seohyun was appointed as the Goodwill Ambassador at the United Nations Pavilion Expo 2012.[47][48] In 2013, as an effort to help less-fortunate students with their studies, she donated 100 million Korean won to Dongguk University. Seohyun expressed, “I learned many things while studying at Dongguk. I wished to be of help to students who have a big dream, and yet don’t have a choice but to give up education because of family matters.”[49] In 2014, Seohyun participated in the charity concert “Happy Day,” donating some of her possessions in order to raise funds in support of Syrian refugee children.[50][51]